The output powers, efficiencies, and lifetimes of short-wavelength ultraviolet light-emitting diodes (UV LEDs), i.e., LEDs that emit light at wavelengths less than 350 nm, based on the nitride semiconductor system remain limited due to high defect levels in the active region. These limitations are particularly problematic (and notable) in devices designed to emit at wavelengths less than 270 nm. Most development effort has been carried out on devices formed on foreign substrates such as sapphire where defect densities remain high despite innovative defect reduction strategies. These high defect densities limit both the efficiency and the reliability of devices grown on such substrates.
The recent introduction of low-defect, crystalline aluminum nitride (AlN) substrates has the potential to dramatically improve nitride-based optoelectronic semiconductor devices, particularly those having high aluminum concentration, due to the benefits of having lower defects in the active regions of these devices. For example, UV LEDs pseudomorphically grown on AlN substrates have been demonstrated to have higher efficiencies, higher power and longer lifetimes compared to similar devices formed on other substrates. Generally, these pseudomorphic UV LEDs are mounted for packaging in a “flip-chip” configuration, where the light generated in the active region of the device is emitted through the AlN substrate, while the LED dies have their front surfaces bonded to a polycrystalline (ceramic) AlN submount. Because of the high crystalline perfection that is achievable in the active device region of such devices, internal efficiencies greater than 60% have been demonstrated. Unfortunately, the photon-extraction efficiency is often still very poor in these devices, ranging from about 4% to about 15% achieved using surface-patterning techniques.
For several reasons, the photon extraction efficiency from short-wavelength UV LEDs is poor compared to visible LEDs. Thus, the current generation of short-wavelength UV LEDs has low wall-plug efficiencies (WPE) of, at best, only a few percent, where WPE is defined as the ratio of usable optical power (in this case, emitted UV light) achieved from the diode divided by the electrical power into the device. The WPE of an LED can be calculated by taking the product of the electrical efficiency (ηel), the photon extraction efficiency (ηex), and the internal efficiency (IE); i.e., WPE=ηel×ηex×IE. The IE itself is the product of current injection efficiency (ηinj) and the internal quantum efficiency (IQE); i.e., IE=ηinj×IQE. Thus, a low ηex will deleteriously impact the WPE even after the IE has been improved via the reduction of internal crystalline defects enabled by, e.g., the use of the AlN substrates referenced above as platforms for the devices.
Several issues can contribute to low photon-extraction efficiency. First, even the highest-quality AlN substrates available generally have some absorption in the UV wavelength range, even at wavelengths longer than the band edge in AlN (which is approximately 210 nm). This absorption tends to result in some of the UV light generated in the active area of the device being absorbed in the substrate, hence diminishing the amount of light emitted from the substrate surface. Additionally, UV LEDs suffer because approximately half of the generated photons are directed toward the p-contact and absorbed by the p-GaN of that contact. Even when photons are directed toward the AlN surface, only 9.4% can escape from an untreated surface due to the large index of refraction of the AlN, which results in a small escape cone. Additional photons are lost on their way to the exit surface due to absorption in the AlN wafer. These losses are multiplicative and the average photon extraction efficiency is only about 2.5%.
In typical LED fabrication, the large difference in the index of refraction between the LED structure and air (and resulting lack of photon extraction) can be greatly ameliorated by using an encapsulant with an intermediate index of refraction. Specifically, many conventional designs feature a “dome” of the encapsulant material disposed over and at least partially surrounding the LED (and subsequently cured by a thermal treatment). The encapsulation increases the critical angle of total internal reflection through the top surface of the semiconductor die, which has led to significant improvements in photon-extraction efficiency for visible LEDs.
To further improve photon-extraction efficiency, attempts have been made to attach optical elements to LEDs using either an encapsulant or an adhesive. An advantage of utilizing such an optical element is that the light emitted by the diode may be directed outward in a more precise way (i.e., as defined by the shape and properties of the optical element). However, optical elements and LEDs generally have different coefficients of thermal expansion, which may result in damage to the LED or the bonding material as the LED heats up during operation. Thus, generally quite thick encapsulant layers have been utilized in order to mitigate the effects of this thermal-expansion mismatch and prevent propagation of thermal expansion mismatch-induced strain between the LED and the optical element.
Unfortunately, LED encapsulants and adhesives are generally organic and/or polymeric compounds featuring carbon-hydrogen bonds (and/or other interatomic bonds) that are easily damaged by UV radiation, leading to degradation of the encapsulant or adhesive. The degradation is particularly severe with exposure to UVC radiation (i.e., radiation at wavelengths less than 300 nm). Thus, using an encapsulant to improve photon extraction is typically ineffective with UV LEDs. And although UV-resistant encapsulants have been developed, even these compounds exhibit degradation upon exposures far less than the desired service lifetime of UV LEDs. For example, the Deep UV-200 encapsulant available from Schott North America, Inc. of Elmsford, N.Y., exhibits a 15% drop in transmittance for 300 nm light after only 1000 hours of exposure.
Thus, there is a need for an easily implementable approach to effectively increase the photon-extraction efficiency from UV LEDs that overcomes the lack of stable encapsulants that are transparent to UV radiation, particularly UVC radiation. Such an approach would desirably enable high transmittance and reliability of UV LEDs without significant degradation over the intended service lifetime of these devices, e.g., approximately 10,000 hours or even longer.